1. Field of the Invention
This invention relates to methods and compositions for removal or inactivation of hydrogen sulfide or soluble sulfide ions from various fluids used in various hydrocarbon recovery or mineral mining operations in subterranean formations. The invention is also applicable to removal of hydrogen sulfide or soluble sulfide ions from other fluids such as fluids produced in such operations from a subterranean formation, and to other fluids that contain hydrogen sulfide such as fluids in sewage systems. The advantages of the invention are particularly appreciated with high pH fluids.
2. Description of Relevant Art
Hydrogen sulfide in fluids is well known to be corrosive to pipes and other containers of the fluids and to many other surfaces in contact with the fluids. Hydrogen sulfide is also a known environmental pollutant and a health risk to persons exposed to it. Low concentrations of hydrogen sulfide irritate conjunctiva and mucous membranes and cause headaches, dizziness, nausea and lassitude. Exposure to high concentrations can result in death.
In drilling some subterranean formations, and often particularly those bearing oil or gas, hydrogen sulfide accumulations are frequently encountered. The drilling fluid or mud brings the hydrogen sulfide to the surface. Such sulfide in the drilling fluid is problematic for the reasons noted above. Generally, to protect the health of those working with the drilling fluid and those at the surface of the well, conditions are maintained to ensure that the concentration of hydrogen sulfide above the fluid, emitted due to the partial pressure of the gas, is less than about 15 ppm. The partial pressure of hydrogen sulfide at ambient temperature is a function of the concentration of sulfide ions in the fluid and the pH of the fluid. To ensure that the limit of 15 ppm is not exceeded even for the maximum sulfide concentration that may be encountered in a subterranean formation, the pH of the drilling fluid is typically maintained at a minimum of about 11.5. Also, to prevent the soluble sulfide concentration in the fluid from becoming excessive, action is routinely taken to remove sulfide from the fluid.
Various methods, techniques and compositions have been used for removing hydrogen sulfide from such fluids. U.S. Pat. No. 4,008,775, issued Feb. 22, 1977, to Fox, teaches a method of scavenging hydrogen sulfide from drilling mud using porous iron oxide particles having a composition of substantially Fe3O4 and having a surface area at least ten times that of magnetite particles of equal size, the greater part of which are no longer than 60 microns.
U.S. Pat. No. 4,756,836, issued Jul. 12, 1988, to Jeffrey et al. teaches decreasing hydrogen sulfide entrained in a drilling mud by adding iron chelate to the mud at the wellhead and circulating the mud in the well being drilling with the mud, allowing the hydrogen sulfide in the mud to be exposed to the iron chelate for conversion of the hydrogen sulfide into elemental sulfur. The chelating agents taught are ethylenediaminetetraacetic acid (EDTA), hydroxethylethylenediaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA), and diethylenetriaminepentaacetic acid (DTPA). Claimed advantages of this invention are said to be that the iron chelate is regenerated by oxygen at the surface and that the iron scavenges oxygen in the mud stream to cut down oxygen assisted corrosion of the drill stem.
This patent to Jeffrey et al. further teaches that whether the iron is supplied in the Fe (II) or Fe (III) form, exposure to oxygen at some point in the mud flow changes the form to Fe (III) to prepare the chelate for hydrogen sulfide conversion. Oxygen exposure in an aerated mud pit or in the shale shaker or by another oxygen source is said to aid regeneration of the iron chelate. While iron (III) is known to readily chelate with EDTA, NTA and HEDTA and DTPA, such complexes have limited stability at high pH. Iron in these complexes is well known to tend to precipitate out as ferric hydroxide at a pH greater than 9. For example, manufacturers of these chelates typically quote stability or effectiveness as an Fe (III) chelate, of NTA at pH 1-3, DPTA at pH 1-7, EDTA at pH 1-6, and HEDTA at pH 1-9. At pH higher than such ranges, these chelating agents lack ability to stabilize the iron against precipitation as the hydroxide. For effective use as a scavenger according to the teachings of Jeffrey invention of U.S. Pat. No. 4,756,836, the iron must stay in chelated form. Further, the multivalent nature of iron III is likely to cause crosslinking of polymers in a polymer based drilling mud, leading to gelation and interference with the rheology of the fluid.
U.S. Pat. No. 6,365,053 B1, issued Apr. 2, 2002 to Sunde, et al. teaches a method of removing hydrogen sulfide from drilling mud using a relatively sparingly soluble divalent environmentally acceptable iron salt in the drilling mud. The preferred such divalent iron salt taught is iron oxalate. The hydrogen sulfide in the mud is said to react with the iron salt to form iron sulfide.
U.S. Pat. No. 6,746,611 B2, issued Jun. 8, 2004 to Davidson, teaches an environmentally friendly method of removing hydrogen sulfide or hydrogen sulfide ions from fluids having a pH in excess of about 9 and as high as a pH of 12 or higher using iron chelating agents having stability at such high pH. The preferred chelating agents taught are ferrous gluconates which are added to the fluid in sufficient quantities to form iron sulfide with the sulfide ion. The iron chelating agent is mixed with the fluid and an iron sulfide is formed.
Using ferrous gluconate to remove hydrogen sulfide from drilling fluids as taught by Davidson has become well known and accepted, as ferrous gluconate is an effective sulfide scavenger that does not impair the properties of the drilling fluid to which it is added. Ferrous gluconate is also fully biodegradable and, as a common dietary supplement, is not considered environmentally incompatible.
Ferrous gluconate is relatively expensive, however, and thus there is a continuing need for environmentally compatible alternatives.